One of well-known mass-analyzing methods using an ion trap mass spectrometer or other apparatuses is an MS/MS analysis (or tandem analysis). In a general MS/MS analysis, an ion having a specific mass number (mass-to-charge ratio m/z) is first selected as a precursor ion from an object to be analyzed. Next, the precursor ion thus selected is dissociated by a collision induced dissociation (CID) process to produce product ions. After that, the product ions are mass analyzed to obtain the information on the mass number of the product ions and desorbed ions, and based on the information, the composition and chemical structure of the target sample molecule are deduced.
In recent years, samples having larger molecular weight than before are becoming analyzed with such an apparatus, and their chemical structure (composition) also tends to become more complicated. Hence, depending on the quality of the sample, ions are often not dissociated to have a sufficiently small mass by only a one-stage dissociation operation. In such cases, an MSn analysis may be performed in which a dissociation operation is repeated more than once and the product ions finally generated are mass analyzed (refer to Japanese Unexamined Patent Application Publication No. H10-142196 and Japanese Unexamined Patent Application Publication No 2001-249114 for example). The aforementioned MS/MS analysis is an MSn analysis in the case where n=2.
In such an MSn analysis, the candidates for the molecular structure and composition of a target substance included in the original sample are basically narrowed down using both the composition formula by the combination of the elements deduced from the precursor ion's mass number and the combination of the elements deduced from the product ion's mass number. For a substance having a large molecular weight such as protein, in the case where the number of the dissociation process is relatively small such as MS2 and MS3, it is difficult to determine the composition formula since the number of the deduced composition formula's candidates is large. However, if the number of the dissociation process dissociation process is increased such as MS4 and MS5, the deduced composition formula's candidates become significantly narrowed down.
For example, in a prior art analysis, it is presumed that the composition formula of a target substance is deduced based on the result obtained by an MS analysis without a dissociation operation (i.e. an MS spectrum), and as a result, five composition formula candidates are lined up. These five composition formula candidates are displayed on a display window in a list format as illustrated in FIG. 8(a) for example. In practice, a, b, c, d, and e in FIG. 8 are each an appropriate number. Next, one precursor ion is selected using the MS spectrum to perform an MS2 analysis, and using the MS2 spectrum which is the analysis result, one ion is furthermore selected as a precursor ion to perform an MS3 analysis to obtain the MS3 spectrum. It is presumed that as the result of performing the refinement of the composition formulae using the information on the peaks appearing on the MS2 spectrum and MS3 spectrum, composition formula candidate number 3 in FIG. 8(a) is excluded and four composition formula candidates remain. This result is displayed on a display window in a list format as illustrated in FIG. 8(b) for example as in the case of FIG. 8(a).
Looking at the aforementioned displays, a person in charge of the analysis can visually and easily check the composition formula candidates remaining at the point in time of the MS3 analysis. However, it is difficult to intuitively know which composition formula candidates are excluded from the result of the MS analysis in the refining process as described earlier. In the example of FIGS. 8(a) and 8(b), the constituent elements consist of only three kinds; carbon (C), hydrogen (H), and oxygen (O). If the kinds of the constituent elements are many, it is more difficult to know it.
The refining process for the composition formula candidates as previously described is automatically performed. Therefore, in some cases a person in charge of the analysis wants to confirm whether or not the refining process is precise and possesses high reliability. This is because, in the case where the refinement of the composition formula candidates is performed based on the information of the peak whose relative intensity is significantly low and its reliability is thought to be low, the reliability of the refining process itself can be thought to be low. However, with the conventional data processors, confirming which peak's information on the MSn spectrum is used as a base to exclude the composition formula candidates is not easy although possible: a complicated operation is required and the operability is not good.